As a method of electrically connecting a semiconductor part or semiconductor parts to a circuit substrate, there is the flip-chip-bonding (FCB) method in which connection pads (electrode pads) of both are connected to one another by using a solder. As one of the FCB technology, there is a method referred to as (pre-coating method) in which a bump is provided for each of connection pads of semiconductor parts, and a solder layer is preliminarily provided on connecting pads of the circuit substrate. FIGS. 1A to 1C are schematic views for the explanatory illustration of the pre-coating method. Namely, a semiconductor part 1 provided with a bump 2 and a substrate 3 having thereon a connection pad 4 on which a solder layer 5 is provided are prepared (FIG. 1A). Positioning or aligning of the bumps 2 and the connection pads 4 is then performed and thereafter, both are brought into contact with one another (FIG. 1B). Heating is then applied to the solder layer 5 on the connection pad 4 to melt the solder thereby connecting bumps 2 and the connection pads 4 by the solder (FIG. 1C).
There are some problems in the conventional method illustrated in FIG. 1 as described below. When the connection pas 4 is comprised of copper (Cu) and when the material of the solder is an alloy of tin and silver (Sn—Ag), an intermetallic compound (Sn—Cu) is formed between the tin (Sn) contained in the solder and the copper (Cu) constituting the connection pad 4 due to heating (pre-coating heat) which is applied during provision of the solder layer 5 on the connection pad 4. As a result, the Sn atom in the solder is decreased while increasing the rate of the silver (Ag) therein, and therefore, the melting point of the solder goes up. Incidentally, even with the Sn—Pb eutectic solder made of tin (Sn) and lead (Pb), in the case where the ratio of the eutectic point of 63% Sn and 37% Pb changes, the melting point similarly goes up. In the case of the Sn—Ag system solder free of lead (Pb), this phenomena of increasing of the melting point is specifically noticeable.
When the melting point of the solder rises, it is necessary that a heating temperature for melting the solder shown in FIG. 1C must be increased. As a result, a difference in the extension (the thermal expansion) between a substrate and a semiconductor part, which is caused by a difference of the thermal expansion coefficient between the substrate and the semiconductor part increases. Therefore, a positional discrepancy between the stud bump 2 and the connection pad 4 as well as a flaw due to the curving of the substrate 3 occur. Further, the hardness of the crystal, i.e., the Ag3Sn crystal which is produced due to the twice melting of the solder upon pre-coating and upon soldering for the connection is very high and therefore, a crack is apt to occur in the hardened solder after melting thereof.
In a conventional method for avoiding the melting of the solder at the pre-coating, there has been provided a paste-transcription method. FIGS. 2A to 2C are schematic views illustrating the conventional paste-transcription method. Namely, a bump 2 of a semiconductor part 1 is immersed in a paste 7 (FIG. 2A). The solder paste 7 is a mixture of a solder particles and a flux having viscosity. Due to the viscosity of the flux, the solder paste 7 is attached to the surface of the bump 2 of the semiconductor part 1. Then, the bump 2 to which the solder paste 8 is attached is positionally aligned on the connection pad 4 on the substrate 3 (FIG. 2B). Thereafter, the bump 2 is heated, to melt the solder paste 8 so that the bump 2 and the connection pad 4 are connected together by the hardened solder 9 (FIG. 2C).
In the paste transcription method of FIG. 2, the melting of the solder is permitted to occur only once and accordingly, it is possible to mitigate the problems caused by the generation of the Sn—Cu intermetallic compound or the Ag3Sn. Nevertheless, there are problems as described below with the paste transcription method. That is to say, when the viscosity of the flux in the solder paste is high, at the time of attaching the solder paste to the bumps, a continuity of the solder paste might occur while forming a bridging of the solder paste between neighboring bumps. On the contrary, when the viscosity of the flux in the solder paste is low, an amount of solder paste attaching to each bump is small. Therefore, in the former case, a short-circuiting might occur between the neighboring bumps after the melting of the solder, while in the latter case, a reduction in the strength of solder connection might take place causing electric disconnection. Consequently, both cases will eventually result in defective solder connection.
Japanese unexamined Patent Publications (Kokai) 06-188289 and 2000-232129 disclose such a method that electric connection between a semiconductor part and a circuit substrate is carried out without meting of solder upon doing the connection. In the disclosed prior art technique of these two publications, a contact pin on the substrate is permitted to pierce a ball-like solder bump on the semiconductor part to establish electric connection between both. However, since the contact pin is placed in a state of merely piercing the solder ball, strength of the connected portion is necessarily small, and an electric contact resistance in the connected portion is larger than the case where melting of the solder is carried out. Further, the prior art disclosed in the above-mentioned two publications takes the way of preliminarily providing a solder bump on an electrode pad of the semiconductor part, and accordingly is different from the conventional methods of FIGS. 1 and 2 in which a solder layer is preliminarily provided on a circuit substrate side.